Valspodar (PSC 833) is a new immunosuppresive and new nephrotoxic cyclosporine analog which is a potent inhibitor of P-glycoprotein. Pgp is expressed in a high percentage of colon cancers and may be an important mechanism of resistance to paclitaxel. The goal of this study was to access the remission rate of paclitaxel in PSC in refractory/metastatic colorectal cancer.